Investment cast article and method of production thereof

ABSTRACT

A method for manufacturing an investment cast article is disclosed. The method comprises positioning an insert relative to a sacrificial form, the insert comprising a first portion and a second portion, the first portion being disposed inside the sacrificial form and the second portion being disposed outside the sacrificial form. The sacrificial form and the second portion of the insert are coated with a material, forming a shell surrounding the sacrificial form and the second portion of the insert. The sacrificial form is evacuated from the shell, followed by introducing molten metal into the evacuated shell, thereby providing a metal form, and resulting in the first portion of the insert being disposed and bonded inside the metal form. The shell is removed from the metal form and the second portion of the insert, thereby resulting in the metal form with the first portion of the insert disposed and bonded therein.

BACKGROUND OF THE INVENTION

The present disclosure relates generally to investment casting methods,and particularly to investment casting of turbine airfoils.

A gas turbine airfoil operates in a high temperature environment.Different parts of the airfoil are subject to different stresses andtemperatures. Accordingly, the application requirements of the differentparts of the airfoil may conflict with one another. The turbine airfoilis often made of a single material. Selection of the single material toensure that all parts of the airfoil will have adequate durability is achallenge that includes a substantial degree of engineering.

The material choice tends to be a balance between all the requiredmaterial characteristics of the airfoil. More specifically, differingapplication requirements of local areas of the airfoil typically requirematerial properties different than those of the base material. Forexample, some areas such as the airfoil tip region or trailing edge,operate at higher temperatures, and will therefore benefit from the useof materials that provide increased oxidation resistance. Other areas,such as the tip shroud contact surfaces or angle wing surfaces willbenefit from the use of hard materials to reduce wear. Additionally,strength, creep fatigue, and cyclic fatigue are factors that areconsidered as part of the material selection.

Turbine airfoils that operate in gas turbines may be manufactured usingan investment casting process. The casting material is often a highstrength alloy of nickel, cobalt, or a combination thereof. The specificmaterial choice is an engineering choice to ensure that the airfoil hasthe desired life given the different stresses and temperatures to whichthe part may be subjected in use.

For parts of the airfoil that require different material properties thanthe base alloy, the general past practice has been to use coatings (suchas wear and oxidation resistant coatings) or to use channels within theairfoil to provide preferential cooling. Another approach is to attachsurfaces having material properties selected for specific airfoillocations onto the airfoil. Coatings increase manufacturing cost andcycle time and may have a finite durability. Preferential cooling maylead to reduced engine performance. Because of difficulties associatedwith welding to the airfoil base materials, until recently brazing wasthe only option to attach surfaces to an airfoil. Welding advances haveprovided some options to weld otherwise unweldable alloys. However, theuse of attached surfaces increases manufacturing cost and cycle time.Accordingly, there is a need in the art for an investment casting methodthat overcomes these drawbacks.

BRIEF DESCRIPTION OF THE INVENTION

An embodiment of the invention includes a method for manufacturing aninvestment cast article. The method comprises positioning an insertrelative to a sacrificial form, the insert comprising a first portionand a second portion, the first portion being disposed inside thesacrificial form and the second portion being disposed outside thesacrificial form. The sacrificial form and the second portion of theinsert are coated with a material, forming a shell surrounding thesacrificial form and the second portion of the insert. The sacrificialform is evacuated from the shell, followed by introducing molten metalinto the evacuated shell, thereby providing a metal form, and resultingin the first portion of the insert being disposed and bonded inside themetal form. The shell is removed from the metal form and the secondportion of the insert, thereby resulting in the metal form with thefirst portion of the insert disposed and bonded therein.

Another embodiment of the invention includes a gas turbine airfoilproduced according to the method of positioning an insert relative to asacrificial form, the insert comprising a first portion and a secondportion, the first portion being disposed inside the sacrificial formand the second portion being disposed outside the sacrificial form. Thesacrificial form and the second portion of the insert are coated with amaterial, forming a shell surrounding the sacrificial form and thesecond portion of the insert. The sacrificial form is evacuated from theshell, followed by introducing molten metal into the evacuated shell,thereby providing a metal form, and resulting in the first portion ofthe insert being disposed and bonded inside the metal form. The shell isremoved from the metal form and the second portion of the insert,thereby resulting in the metal form with the first portion of the insertdisposed and bonded therein. One or more of the second portion of theinsert and the metal form is machined, thereby providing the turbineairfoil.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the exemplary drawings wherein like elements are numberedalike in the accompanying Figures:

FIG. 1 depicts in pictorial form an exemplary embodiment of a method forinvestment casting in accordance with embodiments of the invention;

FIG. 2 depicts in flowchart form an exemplary embodiment of a method forinvestment casting in accordance with embodiments of the invention;

FIG. 3 depicts in flowchart form a generalized exemplary embodiment of amethod for investment casting in accordance with embodiments of theinvention;

FIG. 4 depicts a front perspective view of an exemplary embodiment of aturbine airfoil manufactured in accordance with embodiments of theinvention; and

FIG. 5 depicts a front view of an exemplary embodiment of a turbineairfoil manufactured in accordance with embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the invention provides a method to produce investmentcast parts that have inserts embedded within the cast structure. Byincorporating inserts, a part produced by the investment cast processmay have multiple, optimized material properties, strategically locatedto provide desired application characteristics such as increasedresistance to localized wear, oxidation, corrosion, and elevatedtemperature exposure.

Another embodiment of the invention optimizes the material properties inlocal areas of investment cast turbine airfoils using cast preforms, orinserts. The inserts are made of specific materials havingcharacteristics suited to the environmental exposure of the particular,localized portion of the airfoil in which the insert is placed. Thesepreforms can be made from different materials than the base airfoilalloy. Multiple preforms of different materials are contemplated on asingle airfoil. The preforms are joined to the airfoil during theinvestment casting process and become an integral part of the airfoil.The joining process can involve one or both of a mechanical bond and ametallurgical bond.

Further, it is contemplated that an embodiment of the invention willprovide a reduction in manufacturing steps. Following the investmentcasting of an airfoil, the current practice to attach a surface to thebase material is to machine the airfoil location to which the surfacewill be applied, braze or weld the surface in place, machine thesurface, and then heat treat the assembly. An embodiment of theinvention provides an investment cast airfoil with a cast-in secondarysurface, thereby requiring only a single machining operation.

As a result of the investment casting process, the cast preforms becomean integral part of the airfoil casting. The airfoil then has differentmaterial properties and/or structures to provide optimizedcharacteristics tailored to the application requirements of the specificregions of the part. In this way, subsequent coatings may be eliminated,as well as the need to provide local cooling flow.

Referring to FIG. 1 in conjunction with FIG. 2, an embodiment of anexemplary investment casting process is depicted. A pictorial chart 100and a flowchart 600 depict exemplary steps to provide an investment castpart 250, such as a turbine airfoil for example, containing a machinedinsert 260, which is machined subsequent to casting. In an embodiment,the machined insert 260 provides material properties suited toapplication requirements that relate to the location of the machinedinsert 260 within the airfoil.

The process begins with locating 610 a die 200, the die 200 having acavity 205. The next step is positioning 615 an insert 210 relative tothe die 200. The insert 210 has a first portion 215 and a second portion220, each portion 215, 220 including a mechanical retention feature 216,221 to provide alignment and retention of the insert 210, as will bedescribed further below. The cavity 205 of the die 200 is larger thanthe first portion 215 of the insert 210, thereby allowing the firstportion 215 of the insert 210 to be disposed inside the cavity 205.

The next process step is positioning 615 the insert 210 such that thefirst portion 215 of the insert 210 is disposed inside the cavity 205,and the second portion 220 of the insert 210 is disposed outside thecavity 205, and captivated by the die 200. In an embodiment, theposition of the insert 210 shall be selected based upon the materialcharacteristic requirements of the application for the investment castpart 250. After the insert 210 has been properly positioned 615 withinthe die 200, introducing 620, such as injecting for example, moltenfiller, such as wax or plastic for example, into the cavity 205 createsa sacrificial form 225. In an embodiment, the molten filler will bebonded to the first portion 215 of the insert 210. The mechanicalretention feature 216 is configured to secure the retention and positionof the insert 210 relative to the sacrificial form 225.

Following the introducing 620 molten filler into the die 200, the methodproceeds by removing 625 the sacrificial form 225 and the insert 210from the die 200. The second portion 220 of the insert 210 is disposedoutside of the sacrificial form 225, while the first portion 215 isdisposed and bonded inside the sacrificial form 225, as described above.

Next is coating 630 the sacrificial form 225 and the second portion 220of the insert 210 with a refractory material, such as powdered silica.In an embodiment, the sacrificial form 225 and the insert 210 are dippedin a slurry, which coats the sacrificial form 225, forming a skin. Theskin is dried and the process of dipping and drying is repeated untilthe refractory material forms a shell 230 thick enough to provide thenecessary structural support for the molten metal, as will be discussedfurther below. The material forms the shell 230 surrounding thesacrificial form 225 and the second portion 220 of the insert 210. Aftersufficient time to allow the shell 230 to dry and solidify, heat isapplied to melt the filler, thereby evacuating 635 the sacrificial form225 from the shell 230. Upon evacuation 635 of the filler, the geometryof the sacrificial form 225 is contained by the interior of the emptyshell 230. Similar to the mechanical retention feature 216 of the firstportion 215, it may be appreciated that the mechanical retention feature221 of the second portion 220 is configured and disposed to secure theretention and position of the insert 210 relative to the shell 230. Inthis way, both mechanical retention features 216, 221 secure theposition of the insert 210 relative to the investment cast part 250,allowing the position of insert 210 relative to the investment cast part250 to remain fixed throughout the various steps of the process 100,600.

While an embodiment of the invention has been described using arefractory material of powdered silica, it will be appreciated that thescope of the invention is not so limited, and that the invention mayalso apply to shells made from other materials such as, but not limitedto, plaster of paris, alumina-silicate, and ethyl silicate, for example.While an embodiment of the invention has been described depicting aninsert with a bulb-shaped mechanical retention feature, it will beappreciated that the scope of the invention is not so limited, and thatthe invention may also apply to inserts using alternate means ofmechanical retention, such as a thread, a knurl, a groove, and a step,for example.

In an embodiment, the shell 230 is pre-heated to drive off any remainingfiller material, and to prevent possible cracks as a result of thermalshock from the high temperature of the molten metal. Preheating isfollowed by introducing 640 molten metal into the evacuated shell 230,thereby providing a metal form 240. It will be appreciated that themultiple layers described above are configured to provide the shell 230with the appropriate structural strength, capable to support the weightsand forces resulting from the introduction 640 of the molten metal. Theintroduction 640 of the molten metal may occur while the shell 230 andthe molten metal are subject to a vacuum, in order to extract evolvedgasses and reduce oxidation, thereby increasing the quality of the metalform 240. Subsequent to introducing 640 molten metal, the first portion215 of the insert 210 will be bonded inside the metal form 240. The bondbetween the metal form 240 and the first portion 215 may be one or bothof a mechanical bond, as provided by the mechanical retention feature216, and a metallurgical bond, resulting from diffusion between thematerial of the insert 210 and the material of the metal part 240.

After sufficient time to allow the metal form 240 to cool, the next stepis removing 645 the shell 230 from the metal form 240 and the secondportion 220 of the insert 210. In an embodiment, the shell 230, which istypically brittle, may be removed via mechanical vibration,high-pressure steam, chemical cleaners, or a combination thereof. Otherremoval methods may be employed while still keeping within the scope ofthe invention.

It will be appreciated that the geometry of the metal form 240, which isprovided by the interior of the shell 230 from which the sacrificialform 225 has been evacuated, is directly related to the geometry of thedie 200 that was used to produce the sacrificial form 225. Factors suchas thermal expansion and contraction of the filler, the shell material,and the metal will affect the final metal form 240 geometry, and aretherefore considered in the configuration of the die 200.

Because the insert 210 is disposed bonded within the metal form 240,there is no longer a need for the mechanical retention feature 221 ofthe second portion 220. Therefore, subsequent to the removing 645 of theshell 230, machining 650 the second portion 220 of the insert 210 may bedone to remove the mechanical retention feature 221, providing amachined insert 260. In an embodiment, the machined insert 260 shall beconfigured to provide the geometric tolerances and features required bythe application in which the investment cast part 250 is used. The metalform 240 may also be machined 650 to provide the geometric tolerancesand features of the investment cast metal part 250 required by theapplication.

A more general sense of the method depicted in FIG. 2 by flowchart 600may be represented by the method depicted in FIG. 3 by flowchart 700,where like elements are numbered alike. In the flowchart 700,positioning 621 of the insert 210 relative to the sacrificial form 225may occur without the need to create the sacrificial form 225 within adie 200, as described above. For example, the sacrificial form 225 maybe provided as a machined wax or plastic form, and the insert 210 may beembedded into the sacrificial form 225 by vibratory or other suitableaction.

Referring now to FIGS. 4 and 5, an exemplary embodiment of a gas turbineairfoil 300 is depicted. Using the process 100, 600 described above, itis contemplated that subsequent to the machining 650 of one or more ofthe second portion 220 of the insert 210 and the metal form 240, theturbine airfoil 300 may be produced.

In an embodiment, various locations of the gas turbine airfoil 300 mayutilize the insert 210 to provide improved material characteristics.Between FIGS. 4 and 5, a tip shroud edge 305, tip shroud contactsurfaces 310, an airfoil tip 315, a leading edge 320, a trailing edge325, a platform edge 330, and angle wings 335 are depicted. It will beappreciated that each location described above as part of the airfoil300 has a corresponding position within the cavity 205 of the die 200configured to produce the sacrificial form 225. Further, it will beappreciated that each such position of the die 200 may be configured toreceive the insert 210. Accordingly, the positioning 615 the insert 210into the die 200 may include positioning 615 one or more inserts 210 atpositions of the die 200 corresponding to the airfoil 300 locationsdescribed above. In an embodiment, the first portion 215 of the machinedinsert 260, disposed inside the airfoil 300, is retained by one or moreof a mechanical bond and a metallurgical bond.

In an embodiment of the invention, it is contemplated that each locationdescribed above may utilize an insert 210 comprising materials that aresuited for the conditions to which that location is exposed. It iscontemplated that the machined insert 260 at the airfoil tip 315 may beconfigured to provide one or more of improved oxidation and wearproperties. It is further contemplated that the machined insert 260 atone or more of the trailing edge 325 and the leading edge 320 may beconfigured to provide improved oxidation properties. The machinedinserts 260 at one or more of the tip shroud contact surfaces 310 andthe angle wing surfaces 335 may be configured to provide improved wearproperties. The machined inserts 260 at one or more of the platform edge330 and the tip shroud edge 305 surfaces are contemplated to beconfigured to provide improved corrosion properties.

Materials that may provide characteristics pertinent to the localizedapplication needs of the gas turbine airfoil 300, such as hightemperature, oxidation, wear, and corrosion resistance include, but arenot limited to, Tungsten Carbide and Molybdenum. Accordingly, in anembodiment, the positioning 615 the insert 210 may include inserts 210made of materials comprising one or more of Tungsten Carbide andMolybdenum. The exact composition of the insert 210 selected for usewill be related to the melting temperature of the casting metal. Forexample, it is contemplated to be beneficial to utilize an insert 210that has a melting temperature greater than the melting temperature ofthe casting material, to avoid melting of the insert 210 in response tothe introducing 640 of the molten casting material to the shell 230. Inan embodiment, pre-heating of the shell 230 may be performed at between1700 to 2000 degrees Fahrenheit (927 to 1093 degrees Centigrade) priorto the introduction of molten metal alloy into the shell 230. It iscontemplated that positioning 615 the insert 210 with plating comprisingone or more of platinum and palladium will reduce oxidation of theinserts 210 at such elevated temperatures. Finally, the introducing 640molten metal is contemplated to include investment casting materialsthat are well suited to the high temperatures present within theapplication of a gas turbine airfoil 300, such as alloys having one ormore of Nickel and Cobalt.

As disclosed, some embodiments of the invention may include some of thefollowing advantages: regions of an investment casting that possessmaterial properties optimized for application requirements; a turbineairfoil with application-specific inserts; a turbine airfoil that may beproduced without coatings or thermal cooling passages; and the abilityto produce a turbine airfoil having application-specific surfaces usingfewer manufacturing steps.

While the invention has been described with reference to exemplaryembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best oronly mode contemplated for carrying out this invention, but that theinvention will include all embodiments falling within the scope of theappended claims. Also, in the drawings and the description, there havebeen disclosed exemplary embodiments of the invention and, althoughspecific terms may have been employed, they are unless otherwise statedused in a generic and descriptive sense only and not for purposes oflimitation, the scope of the invention therefore not being so limited.Moreover, the use of the terms first, second, etc. do not denote anyorder or importance, but rather the terms first, second, etc. are usedto distinguish one element from another. Furthermore, the use of theterms a, an, etc. do not denote a limitation of quantity, but ratherdenote the presence of at least one of the referenced item.

1. A method for manufacturing an investment cast article, the methodcomprising: positioning an insert relative to a sacrificial form, theinsert comprising a first portion and a second portion, the firstportion being disposed inside the sacrificial form and the secondportion being disposed outside the sacrificial form; coating thesacrificial form and the second portion of the insert with a material,the material forming a shell surrounding the sacrificial form and thesecond portion of the insert; evacuating the sacrificial form from theshell; introducing molten metal into the evacuated shell, therebyproviding a metal form, and resulting in the first portion of the insertbeing disposed and bonded inside the metal form; and removing the shellfrom the metal form and the second portion of the insert, therebyresulting in the metal form with the first portion of the insertdisposed and bonded therein.
 2. The method of claim 1, wherein thepositioning the insert further comprises: positioning the insertrelative to a die having a cavity larger than the first portion of theinsert, such that the first portion of the insert is disposed inside thecavity, and the second portion of the insert is disposed outside thecavity and captivated by the die; introducing molten filler into thecavity to create the sacrificial form, resulting in the filler beingbonded to the first portion of the insert; and removing the sacrificialform and the insert from the die, the second portion of the insertdisposed outside the sacrificial form, thereby providing the sacrificialform and the insert.
 3. The method of claim 1, further comprising:machining the second portion of the insert subsequent to the removing.4. The method of claim 1, wherein: the introducing molten metal occurswhile the shell and the molten metal are subject to a vacuum.
 5. Themethod of claim 1, wherein the positioning comprises: positioning theinsert comprising mechanical retention features.
 6. The method of claim1, wherein the manufacturing comprises: manufacturing an investment castgas turbine airfoil.
 7. The method of claim 6, wherein: the positioningcomprises positioning the insert made from a material to providecharacteristics pertinent to localized application needs of the gasturbine airfoil including at least one of temperature, oxidation, wear,and corrosion resistance; the positioning comprises positioning theinsert comprising a plating made from platinum, palladium, or acombination thereof; and the introducing comprises introducing an alloyof nickel, cobalt, or a combination thereof.
 8. The method of claim 6,wherein the positioning further comprises: positioning the insertrelative to a die having a cavity larger than the first portion of theinsert, such that the first portion of the insert is disposed inside thecavity, and the second portion of the insert is disposed outside thecavity and captivated by the die; introducing molten filler into thecavity to create the sacrificial form, resulting in the filler beingbonded to the first portion of the insert; and removing the sacrificialform and the insert from the die, the second portion of the insertdisposed outside the sacrificial form, thereby providing the sacrificialform and the insert.
 9. The method of claim 8, wherein: the die cavityhas a plurality of positions that correspond to an airfoil tip, anairfoil leading edge, an airfoil trailing edge, airfoil tip shroudcontact surfaces, airfoil angle wings, an airfoil platform edge, and anairfoil tip shroud edge; and the positioning comprises positioning oneor more inserts at the positions of the die cavity corresponding to oneor more of the airfoil tip, the airfoil leading edge, the airfoiltrailing edge, the airfoil tip shroud contact surfaces, the airfoilangle wings, the airfoil platform edge, and the airfoil tip shroud edge.10. The method of claim 6, further comprising: machining one or more ofthe second portion of the insert and the metal form.
 11. A gas turbineairfoil produced according to the method of: positioning an insertrelative to a sacrificial form, the insert comprising a first portionand a second portion, the first portion being disposed inside thesacrificial form and the second portion being disposed outside thesacrificial form; coating the sacrificial form and the second portion ofthe insert with a material, the material forming a shell surrounding thesacrificial form and the second portion of the insert; evacuating thesacrificial form from the shell; introducing molten metal into theevacuated shell, thereby providing a metal form, and resulting in thefirst portion of the insert being disposed and bonded inside the metalform; and removing the shell from the metal form and the second portionof the insert, thereby resulting in the metal form with the firstportion of the insert disposed and bonded therein; machining one or moreof the second portion of the insert and the metal form, therebyproviding the turbine airfoil.
 12. The gas turbine airfoil of claim 11,wherein: the insert comprises mechanical retention features.
 13. The gasturbine airfoil of claim 11, wherein: the first portion of the machinedinsert is retained by one or more of a mechanical bond and ametallurgical bond.
 14. The gas turbine airfoil of claim 11, wherein:the airfoil is made from an alloy of nickel, cobalt, or a combinationthereof; and the insert is made from a material to providecharacteristics pertinent to localized application needs of the gasturbine airfoil including at least one of temperature, oxidation, wear,and corrosion resistance.
 15. The gas turbine airfoil of claim 11,comprising: an airfoil tip, an airfoil leading edge, an airfoil trailingedge, airfoil tip shroud contact surfaces, airfoil angle wings, anairfoil platform edge, and an airfoil tip shroud edge; one or moremachined inserts at one or more of the following locations of theairfoil: the airfoil tip; the airfoil leading edge; the airfoil trailingedge; the airfoil tip shroud contact surfaces; the airfoil angle wings;the airfoil platform edge; and the airfoil tip shroud edge.
 16. The gasturbine airfoil of claim 11 wherein: the melting temperature of theinsert is greater than the melting temperature of the metal form.